Seismic surveying is used for identifying subterranean elements, such as hydrocarbon reservoirs, fresh water aquifers, gas injection reservoirs, and so forth. In performing seismic surveying, seismic sources are placed at various locations on an earth surface, sea floor or at a sea surface (or in a wellbore), with the seismic sources activated to generate seismic waves directed into a subterranean structure. Examples of seismic sources include explosives, air guns, or other sources that generate seismic waves.
The seismic waves generated by a seismic source travel into the subterranean structure, with a portion of the seismic waves reflected back to the surface (earth surface, sea floor, sea surface, or wellbore surface) for receipt by seismic sensors (e.g., geophones). These seismic sensors produce signals that represent detected seismic waves. Signals from the seismic sensors are processed to yield information about the content and characteristic of the subterranean structure.
Seismic migration refers to the process of constructing surfaces of subterranean elements that are part of the subterranean structure of interest from recorded seismic data. The process of migration causes generation of an image of a seismic section or volume in the subterranean structure. Migration improves the image of a seismic section or volume by correcting for lateral mis-positioning of dipping seismic reflectors, misplacement of subterranean reflectors due to structural dip, discontinuities, curvature of formations, and other factors.
To perform marine surveying, a sea vessel can be used to tow several streamers, where each streamer includes a number of seismic receivers. Seismic sources can be towed by the same sea vessel, or by different sea vessels sailing parallel courses.
Conventionally, discrete seismic sources are activated in sequence to produce discrete shot gathers. The discrete shot gathers are then migrated to produce discrete images, which are then summed together to form a composite image that represents a subterranean structure of interest. This process involves activating a first seismic source, recording the seismic data due to activation of the first seismic source, then separately activating a second seismic source, recording the resulting seismic data due to the activations of the second seismic source, and so forth. For each particular gather, a given source is activated multiple times to produce different shots, while the given source is moving. A gather is thus a series of measurements made by receivers for different positions of a source. If many seismic sources are used, then the sea vessel that tows the streamers containing the seismic receivers would have to be towed relatively slowly; otherwise, the spacing between shots for a given source can become relatively large.